The growing trend to optical wavelength division multiplexed (WDM) systems has created a need for laser apparatus that can be a source of a plurality of optical signals of well separated and stable frequencies. To this end, it is important that multilongitudinal mode operation at the chosen wavelength be suppressed. Additionally, it is important that a high bitrate modulation of the chosen wavelength be possible. It is also desirable that the laser apparatus be of small size, and preferably largely integrated on a single semiconductive chip.
Generally laser apparatus to be of small size involves the use of a semiconductive chip that is enclosed within an appropriate resonant structure. In one particular form, a waveguide grating router (WGR) is used to provide filter control of the operating wavelength. In such a structure it is desirable to have the filter bandwidth of the WGR as narrow as possible so that at most only a few longitudinal modes of the laser chip are lying in the WGR passband. Ordinarily a relatively large-sized WGR is needed for generating a narrow passband filter bandwidth.
However, the geometrical size of the laser cavity generally should be as small as possible because, to a first order approximation, the laser cavity length determines the maximum speed for data modulation in a long-cavity laser. Additionally the smaller the size of the laser chip, generally the smaller its cost because less semiconductive material is involved and more laser chips can be obtained from a wafer of a given size.
A resonant structure that has been developed for use in gas laser systems is the so-called DiDomenico laser resonator described in a paper in the IEEE Journal of Quantun Electronics, August 1966, pp. 331-322 entitled "Characteristics of a Single-Frequency Michelson-Type He-Ne Gas Laser." This structure is an open-ended resonator that provides longitudinal mode selection and amplitude stabilization in gas lasers.
It comprises a pair of coupled cavities, each enclosing an independently tunable amplifier whose outputs are combined. By choosing slightly different lengths for the two cavities, filter functions similar to those of a Fabry-Perot filter can be realized and the passband bandwidth of the resulting structure can be much narrower than that of either one of the two cavities. The conventional DiDomenico resonant structure includes a set of three mirrors and a beam splitter to form the pair of coupled cavities.